1. Field of the Invention
The invention relates to trapped field magnets formed from high temperature superconductor material. The present invention provides enhanced field strength superconductor magnets. Applications for this basic technology include motors, generators, magnetic clamps, rivet guns, magnetic resonance imaging, magnetic levitation bearings and other applications where enhanced field strength superconductor magnets are useful.
2. Description of the Prior Art
It has long been known that Type II superconductors could be used to replicate externally generated magnetic fields. M. Rabinowitz, et al., Nuovo Cimento Lett., 7, 1 (1973) disclosed low temperature magnetic replicas in 1973. Prior art magnetic replication efforts focused on achieving the fidelity of relatively small fields only at 4.2 K. Rabinowitz was the first to successfully trap a multipole field with high fidelity perpendicular to the axis of a cylinder made of low temperature superconductor such as Pb, Nb or Nb.sub.3 Sn. Rabinowitz also proposed to use a superconductor of simple geometry, i.e. a cylinder or a plate, as a magnetic replica to copy from a template a magnetic field with various complexity.
High temperature superconductors (HTS) were also known to be Type II and capable of trapping magnetic fields. Soon after the discovery of HTS, Weinstein proposed to use them to trap and replicate magnetic fields with additional advantages. See R. Weinstein, et al., Applied Physics Letter, 56, 1475 (1990). Notwithstanding these prior developments, practical applications for HTS trapped field magnets have been limited in several respects. One significant limitation of the prior art is the maximum strength of the trapped field which can be achieved using conventional methods.
HTS have a very high irreversible field B.sub.i which sets the theoretical limit for the maximum field strength B.sub.T achievable. For YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO), B.sub.i is approximately 4 T at 77.degree. K. and &gt;100 T at 4.2.degree. K. when the field is parallel to the c-axis of this compound. It has been expected that B.sub.i could be further raised by high-energy heavy-particle irradiation. According to C. P. Bean, Physics Review Letter 8, 250 (1962), the maximum field strength B.sub.T is proportional to J.sub.c d for an infinite slab of superconductor with a thickness d and critical current density J.sub.c, neglecting the magnetic field effect on J.sub.c. Therefore one needs to enhance J.sub.c and/or d to achieve a large B.sub.T.
Because of the short coherence length of HTS, only irradiation by high-energy particles has been found to be effective in raising the J.sub.c of bulk HTS to date. Researchers I. G. Chen and R. Weinstein, as reported in IEEE Transactions in Applied Superconductivity (1992), have found a four to six-fold enhancement of B.sub.T in bulk YBCO following high-energy proton-irradiation. Irradiation, however, is impractical because it is expensive and leaves the HTS radioactive.
Alternatively, one can increase J.sub.c by lowering the temperature for field trapping. Since J.sub.c is known to increase by a factor of 50 to 100 when cooled from 77 K to 4.2 K, a very strong B.sub.T would be expected with B.sub.i as the only limit. Unfortunately, a flux-avalanche (FA) or large flux jump associated with -thermal instabilities (See E. W. Collins, "Advances in Superconductivity II" (Springer-Verlag, Berlin, 1990; p. 327)) in bulk HTS was recently observed by us. This FA severely restricts the final B.sub.T to approximately 4-5 T at 4.2.degree. K. in an unradiated YBCO bulk sample of dimensions approximately 20 mm diameter by 7 mm thick.
Because of the severely weakened J.sub.c at the grain boundaries in HTS due to their short coherence length, d represents the grain size instead of the sample size of an HTS used for a trapped field magnet. To increase B.sub.T by increasing d, one must grow bulk HTS with large grains. Recently we have succeeded in growing large, single-grain HTS (.about.40 mm diameter.times.15 mm thick). In larger HTS, however, the quality of the grain degrades with increasing d.
Until recently, the record B.sub.T was approximately 2.2 T at 4.2.degree. K. in a cylinder wound with NB.sub.3 Sn tapes kept at 4.2.degree. K. M. W. Rabinowitz and S. D. Dahlgren, Applied Physics Letter 30, 607 (1977). Chen and Weinstein obtained a B.sub.T of approximately 1.42 T at 77.degree. K. at the center of a stack of small YBCO tiles corresponding to a B.sub.T of only 0.7/T at the surface of the stack of YBCO tiles after proton-irradiation, or a much smaller value than 0.7/T prior to proton-irradiation. Sawano, et al., Japan Journal of Applied Physics, 30, L1157 (1991), succeeded in trapping a B.sub.T of approximately 0.72 T at 77.degree. K. in a single grain YBCO disk (44 mm diameter.times.15 mm thick) before irradiation.
Within the inherent limit of B.sub.T &lt;B.sub.i, the most serious obstacle to ultra-high B.sub.T at low temperatures (e.g., .gtoreq.4.2.degree. K.) is FA due to thermal instabilities which increase with the dimensions of the HTS samples. The other obstacle is the degradation of the effective J.sub.c as the size of the bulk HTS increases. For instance, the J.sub.c at 77.degree. K. for a small HTS sample (10.times.0.6.times.0.6 mm.sup.3) is approximately 80.times.10.sup.3 A/cm.sup.2 in contrast to the approximate 6.times.10.sup.3 A/cm.sup.2 for a large one (45 mm diameter.times.15 mm thick). This limitation is attributed to the present difficulties in large-grain growth, e.g. controlling the exact crystal alignment and minimizing the weak links in large samples.